Systemic Sclerosis (SSc) is a multisystem, fibrosing disorder in which vasculopathy, autoimmunity, and inflammation lead to diverse life-altering and life-threatening clinical manifestations. Treatment is typically focused on specific organ involvement and there is no standardized drug to treat the symptoms with significant differences in the therapeutic approaches between experts. Furthermore, the development of new drug is complicated by the heterogeneity of the disease but also by the absence of validated outcome measures. Recent reports have demonstrated that the chronic activation of plasmacytoid dendritic cell (pDCs) and subsequent secretion of both IFN-? and the chemokine CXCL4 is associated with the pathogenesis of SSc. Our preliminary data show that pDCs have a key role in promoting skin fibrosis in a mouse model of scleroderma. We also show that the pattern of TLR expression is altered in the pDCs of SSc patients with high expression of TLR8, a receptor that is absent in pDCs from healthy donors or SLE patients. Signaling though TLR8 induced both IFN and CXCL4. Using mice that we have engineered so they bear human TLR8, we show that TLR8 promotes fibrosis as these mice have exacerbated disease. We will test the hypothesis that pDCs is the critical cell type promoting SSc due to the aberrant expression of TLR8 on pDCs that impacts the response to self-nucleic acids in patients. The project will use two separate mouse models of scleroderma to define the role of pDCs, TLRs and key signaling molecules of the TLR pathway such as PI3K? in the development of disease. We will also dissect the underlying defect that leads to the production of CXCL4 in pDCs from SSc patients and will aim to better understand the interplay between CXCL4 and the IFN response following TLR triggering in patients. To tackle these questions, we are proposing to (1) determine what controls pDCs activation in SSc patients and how these cells promote disease and (2) to characterize the nature of TLR8 stimulation of SSc pDCs and evaluate how CXCL4 and IFN impact TLR8 response. We will also determine whether key signaling molecules induced following TLR triggering are involved in disease progression. The data generated here will provide key understanding of the role of pDCs in skin pathology and the factors that control pDCs activation in SSc patients, thus identifying new ways to manipulate these cells in pathological conditions.